There are currently very few ionisation sources capable of providing a simple mass spectrum for non-polar aliphatic chemicals including alkanes, alkenes and alkynes from a GC inlet. High energy electron impact (electron ionisation) provides considerable fragmentation data but little molecular ion information. Field ionisation and field desorption have also been used for the characterisation of paraffins and olefins within crude oil or fractions thereof. However, the carbon dendrite filaments used in these methods can be very fragile, leading to a lack of robustness and reducing the number of samples that can be analysed before the filaments need replacing. In addition, the carbon dendrites can degrade during the acquisition resulting in irreproducibility in response.
Atmospheric pressure chemical ionisation (APCI) is an ionisation method used in mass spectrometry which utilises gas-phase ion-molecule reactions at atmospheric pressure to produce ions from an analyte which are then analysed by mass spectrometry. APCI is based on a coronal discharge process. There are two main potential ionisation mechanisms: nitrogen charge transfer (producing predominantly a radical cation) and protonation (resulting in a protonated molecular ion). The dominant ionisation process can be defined by the environment, which is ultimately controlled by the user.
Certain types of molecules including aliphatic molecules such as paraffins, isoparaffins and olefins do not consistently ionise by the expected routes. Paraffins for example have been shown to undergo hydride abstraction to form [M−H]+ in addition to formation of nitrogen/carbon/oxygen adducts including [M+N]+ and [M+CH4N2]+ in addition to some useful backbone fragmentation. The result is a complex mass spectrum which may be difficult to interpret without prior knowledge. Results may be inconsistent, as different mass spectrometers or environments may give rise to different spectra for the same compounds. For example, the same type of molecules can undergo different ionisation processes depending upon the gas flow, gas composition and temperature of the environment during the ionisation process. In one example, it has been found that APCI-GC analysis of the alkane hexatriacontane (C36H74) can show three dominant ions in the spectrum including the hydride abstracted [M−H]+ (M−1), in addition to an [M−H+N]+ (M+13) adduct and a [M+NOH2]+ (M+32) base peak. However, the relative abundances of these adducts can vary significantly depending on subtle changes within the source environment (e.g. gas composition, velocity or temperature).
Water and alcohols have been used as modifiers or dopants to induce protonation during atmospheric pressure chemical ionisation of polar molecules. However, the presence of water can further complicate the mass spectra of aliphatic compounds through the formation of adducts. There therefore remains a need to improve the analysis of aliphatic molecules by mass spectrometry.